I believe a short link was proposed which was basically underground junctions about mile and half from Euston (about half mile from tunnel portal at Euston end), near where headhouse is at Primrose Hill.
Would then have been short spurs (actually long enough to hold a 400m train clear of the line to Euston) leading to a single line to join the provision for spurs that are in the open north of St Pancras.
From memory two versions of plan exist, one is cheaper and runs at low speed squeezed in alongside the north London line at Camden before diving into the tunnel spurs, and other is straighter with longer tunnel. Nothing is currently authorised.
I think the general view is that the larger junction tunnels need to be built even if spur is not added for years because it will be very disruptive to Euston services to do later. I have not seen anything that says if these step plate style sections will be built (but then been nothing to indicate the tunnel bore will be reduced here either) as part of the Euston tunnelling announcement.
Its probably worth noting the TBMs were not scheduled to bore whole tunnel anyway, the approach tunnel splits into two at Euston end (basically there are two approaches, one either side on outside, with departure tunnel in middle, to reduce conflicting moves), some of this multi level approach arrangement (where inbound and outbound tunnels at different levels) would be dug by roadheaders not TBMs